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Many demanding applications in the RF and microwave industry require fixed frequency ultra low phase noise frequency sources that provide a high level of performance and reliability. These low noise frequency sources are often comprised of individual quartz-based ovenized crystal oscillators (OCXO) coupled with modular system building blocks such as amplifiers, multipliers, dividers, PLLs and filters that are designed to maintain the low noise integrity of the oscillators and combined to create the desired frequencies and functionality. When required, these components can be integrated into a much smaller footprint to satisfy limited space and weight constraints. These Integrated Microwave Assembly (IMA) products can be custom designed to meet specific needs. Now more standard configurations are also available.

In order to provide fast delivery and excellent performance while minimizing case size, Wenzel Associates has introduced the Multiplied Crystal Oscillator (MXO) Series. This oscillator series of standard integrated assemblies offers world-class performance with the integration of Ultra Low Noise Oscillators and Blue Tops system building blocks in a single machined aluminum case. These ultra low noise sources can be specified at fixed frequencies between 200 MHz and 12 GHz. The versatility offered in the MXO Series products provides quick configuration of previously custom components and makes it an excellent choice for users of high quality frequency sources from VHF to X-band.

Configuration

There are many ways to create a specific frequency of interest within the MXO oscillator series. Size and performance can be considered in order to optimize the MXO for a specific application at a specific frequency. Table 1 shows some of the more common configurations possible. The circuit configuration and options determine the appropriate case configuration.

Figure 1 8 GHz MXO (MXO-8000-J3P) with integrated PLL.

There are six cases available: J1, J1P, J2, J2P, J3 and J3P. The “J” and number specify the output stage and the “P” means that a phase lock loop circuit is included. Figure 1 shows the largest case configuration (J3P) and the block diagram for an 8 GHz MXO with the phase locking option added (MXO-8000-J3P). This drawing details the various circuits and stages required to create the 8 GHz MXO and how they are separated and interconnected. Standard options include phase locking to an external reference, providing a higher output power level or adding filters for better spectral purity.

Each configuration contains the VHF oscillator stage that includes the ultra low noise oscillator and an optional basic x2 stage. The crystal for this oscillator, provided by the company’s Croven Crystals division, is selected based on the final frequency and multiplier configuration needed.

Output Stage 1 (OS1) adds multiplier 1 and its associated circuitry. This multiplier stage contains the lowest noise odd or even order multipliers available from Wenzel. The final output stage case configuration is J1 or J1P at 2.25 × 4 × 1" or 3.445 × 4 × 1" with phase locking circuitry. This configuration can provide outputs to over 1.2 GHz.

Output Stage 2 (OS2) adds multipliers 2A and 2B to multiplier 1. These stages can also include either odd or even order multipliers that can be installed in either of the two chambers available in this stage. This is the final output stage in case configurations J2 and J2P. The package sizes are 3.205 × 4 × 1" and 4.40 × 4 × 1" with phase locking. Adding Output Stage 2 can provide outputs to over 7 GHz.

Output Stage 3 (OS3) also offers two chambers to house two additional circuits, typically x2 multipliers, and is the final stage for case configurations J3 and J3P. Package size is 4.16 × 4 × 1" and 5.355 × 4 × 1" with phase locking. Output Stage 3 contains multipliers that will generate frequencies up to 12 GHz.

Performance

Typical phase noise, harmonics, sub-harmonics and spurious performance for some common MXO configurations are shown in Table 2. An output level of +10 to +13 dBm is fairly standard, but choosing a different amplifier or adding an additional amplifier stage can provide higher output levels. The phase noise performance is the result of Wenzel’s industry leading ultra low noise oscillator and the theoretical 20 log(N) degradation of the multipliers plus a few dB of additional intrinsic noise. The multipliers and other circuits following the oscillator are all designed to minimize additive noise, providing the best phase noise possible. Excellent spectral purity is achieved through the use of several strategically located high quality filters. A +15 VDC supply voltage or optionally, +12 VDC, is required to power the oscillator and other circuits, which is internally regulated to minimize power supply noise.

Figure 2 MXO measured phase noise plots.

Figure 2 shows measured phase noise performance for various MXO configurations at 512 MHz, 1, 2.5, 6.8 and 8 GHz. In each case, the crystal frequency was selected to provide excellent close-in phase noise, and the circuits were optimized to achieve ultra low noise floors. Typical noise floor of the 8 GHz MXO is -133 dBc/Hz. With a 1 GHz MXO, the noise floor is better than -154 dBc/Hz.

Applications

The MXO Series product line provides high quality signal generation solutions to 12 GHz for some of the most demanding applications in the commercial and defense markets. For example, the MXO can be used as a reference oscillator for phase noise testing by universities and research facilities, as a frequency source generator in a linear accelerator or as a stable high performance source as part of a radio astronomy or telecom system. The compact size, light weight, rugged design and exceptional performance of the MXO make it an excellent frequency source for many military applications such as electronic warfare, ground, mobile and satellite communications, and various radar applications. Low-G performance, MIL-screening and testing to MIL-PRF-55310 can be provided as well.